The nanometric scale ploughing friction and wear behaviour of a pyramidal diamond indenter sliding against a face-centred cubic silver (100) surface is investigated by means of parallel molecular dynamic (MD) simulations of nanoindentation followed by nanoscratching. The relationship between the friction coefficient, the hardness and the indenter orientation is studied. The simulations were performed using three different indenter orientations. For each orientation, simulations were performed at an indentation depth of 5 and 10 Å, and a scratching length of 210 Å. In order to study the behaviour of the friction coefficient and the hardness as a function of depth we performed the simulations for one of the orientations at depths of 5, 10 and 30 Å. The simulations show that the friction coefficient is dependent on both the orientation of the indenter and the indentation depth. The results also show that the friction coefficient increases as the depth increases, whereas the contact pressure decreases and the scratch hardness decreases slightly. With a shallow indent of 5 Å, no sub-surface defects were observed beneath the scratch groove, but with the deeper indents of 10 and 30 Å dislocations in the {111} planes are observed beneath the scratch groove. These dislocations propagate in the direction; each dislocation consists of the intersection of stacking faults on two {111} planes and each stacking fault is bounded by two Shockley partial dislocations.

Molecular dynamics simulations are performed to investigate the atomic-scale stick-slip phenomenon of a pyramidal diamond tip inserted into the Ag͑010͒ surface. The mechanisms behind the stick-slip events are investigated by considering sliding speeds between 1.0 and 5.0 ms Ϫ1 and vertical support displacements of 5 and 15 Å. The analysis of the dynamic features of the substrate shows that dislocations are extrinsically linked to the stick events, with the emission of a dislocation in the substrate region near the tip, when slip occurs after stick. For small vertical displacements, the scratch in the substrate is not continuous because the tip can jump over the surface when slipping, whereas at 15 Å, a continuous scratch is formed. The dynamic friction coefficient increases from ϳ0.13 to ϳ0.46 with increasing depth, but the static friction coefficient increases only from ϳ0.32 to ϳ0.54. At the larger depths the tip does not come to a halt during stick as it does for shallow indents. Instead the tip motion is more continuous with stick and slip manifested by periods of faster and slower motion. Although the exact points of stick and slip depend on the sliding speed, the damage to the substrate, the atomistic stick-slip mechanisms, and the friction coefficients are relatively independent of speed over the range of values considered.

Molecular dynamics (MD) simulations of atomic-scale stick-slip have been performed for a diamond tip in contact with the (100) surface of fcc Ag, bcc Fe, Si and H-terminated Si, at a temperature of 300 K. Simulations were carried out at different support displacements between 5 and 15Å. The simulations illustrate the important mechanisms that take place during stick-slip. In particular, for the case of the metals they show a direct link between tip slip events and the emission of dislocations from the point of contact of the tip with the substrate. This occurs both during indentation and scratching. For the case of silicon, no slip events were observed and no sub-surface dislocations were generated underneath the scratch groove. At the deeper support displacement of 15Å the silicon atoms undergo some local phase transformations and the atom co-ordination number varies between 5 and 8, with the majority being 5-fold or 6-fold coordinated. Both the dynamic and the static friction coefficients were found to be higher for Si compared to the corresponding values for H-terminated Si. Comparisons were made between the MD simulations and experimental measurements for indentation on the (100) surface of Si and Al. A good qualitative agreement was observed between the experimental and theoretical results. However in both the cases of Si and metals the MD simulations give a contact pressure under load that is depth dependent and values that are higher than experimental nanohardness values.

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We present results of parallel molecular dynamics (MD) simulations of nanoindentation and nanotribology experiments. The models we have developed describe both the sample and the indenter atomistically and model the effect of the cantilevers in an atomic force microscope (AFM) through the use of springs. We show that the simulations are in good qualitative agreement with experiment and help to elucidate many of the mechanisms that take place during these processes. In particular we illustrate the role that dislocations play both in nanoindentation and also in stick-slip. Further to this we show how real-time visualisation and computational steering have been employed in these simulations to capture the dynamical events that take place.